1. Field of the invention
The present invention relates to a polyester film for a magnetic tape. More particularly, the present invention relates to a polyester film which has high strength in both longitudinal and transverse directions and excellent abrasion resistance and is suitable as a base film of a magnetic tape.
2. Description of the Related Art
Hitherto, as a base film of a long-play video tape, a so-called longitudinal tensilized film has been used. Recently, with the increased need for a magnetic tape with high image quality and high resolution, a particle size of magnetic powder is decreased and magnetic powder particles are highly orientated in a magnetic layer. As a result, the magnetic layer has decreased strength in the transverse direction. In addition, since a thickness of the magnetic tape is decreased after long time recording and therefore tape stiffness is decreased, a spacing loss arises between the tape and a magnetic head so that output power tends to deteriorate and also irregularity of an envelope shape tends to appear. Further, a tape surface is smoothed and the coefficient of friction increases, resulting in tape damage. To solve the above problems, it is highly desired to increase the strength of the magnetic tape in the transverse (widthwise) direction and to provide a so-called both longitudinally and transversely tensilized film.
To produce a both longitudinally and transversely tensilized film, it is necessary to stretch or expand the film in two directions, namely in the longitudinal and transverse directions of the film. As a result, particles tend to fall and powder tends to be by abrasion. That the fallen particles and powder adhere onto the calendering roll, whereby productivity of the magnetic tapes is greatly deteriorated.
To avoid such problems of fallen particles and powder, kinds, particle sizes and amounts of the particles to be added to the film have been examined, but no satisfactory solution has been found.